One indicator of relative position between two radio communication devices is received radio signal strength. Often a number is assigned to this strength and is referred to as the received signal strength indication (RSSI). Most manufacturers who report RSSI generally estimate the received signal power at the antenna either by direct measurement, or digital signal processing, and report a monotonically increasing number with respect to this power. The number is often calibrated to track power linearly and report the value in dBm.
Received signal strength depends on transmit power level, the direct line of sight and reflection path distances between transmitter and receiver, and the phasing of any reflected radio waves received, where the reflected radio waves are referred to as multi-path. As the direct line of sight distance increases for a fixed transmit power, the received amplitude decreases in general, but can vary about in amplitude around this trend due to reflections. In some situations, the direct path is blocked and only reflections are received.
When the direct path is not blocked, relatively large decreases in received signal strength, referred to as signal “nulls” or “fades” can be caused by reflectors at or beyond the first Fresnel zone. Fresnel zones are ellipsoids with transmitter and receiver at the foci and surface defined by all paths that are an odd-multiple of a half-wavelength farther than the direct path between transmitter and receiver, causing cancellation, assuming that there is no phase shift at the reflector.
If the reflector is at an even multiple of a half-wavelength, the direct and reflected waves can constructively interfere (again, assuming no phase shift at the reflector), resulting in a signal “peak” where the received amplitude may be twice as large as the direct path alone. In general, accurate predictions of real situations are difficult, but one can state in general that the actual received signal strength indication can vary by +6 dB to −infinity depending on the reflector configuration. When the direct path is blocked, signal nulls can also occur, and generally RSSI will decrease with distance. Often one reflector dominates, and a simple analysis for direct path fading can be used.
In the prior art, a variety of means have been used to determine relative position between two radio devices. As described earlier, radio signal strength indication has been used to indicate relative location. The RSSI may have a monotonic non-linear relationship. A calibrated mapping could be made for relative distance versus RSSI. Other prior art techniques for relative location include the use of GPS, pulse time delay, and triangulation based on access points.
One advantage of RSSI is that it is usually accessible by software in a radio equipped device, being a measured quantity needed for dynamically optimizing radio system operation. No significant processing or additional components are required like GPS, pulse time delay and triangulation.
However, the use of RSSI to determine relative position may be problematic due to signal nulls resulting from multi-path. There also can be signal peaks due to multi-path, but these seldom have the large changes in signal strength seen by nulls. As a result, there is a need for improved methods and apparatuses for ranging between two radio devices.